Re: BUG: KASAN: use-after-free in dec_rlimit_ucounts

From: ebiederm@xmission.com (Eric W. Biederman)
To: Alexey Gladkov <legion@kernel.org>
Cc: Qian Cai <quic_qiancai@quicinc.com>, Yu Zhao <yuzhao@google.com>,
	linux-kernel@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: BUG: KASAN: use-after-free in dec_rlimit_ucounts
Date: Mon, 27 Dec 2021 09:22:20 -0600	[thread overview]
Message-ID: <87zgomrs2b.fsf@email.froward.int.ebiederm.org> (raw)
In-Reply-To: <20211221130937.yokn25stgjpzax7n@example.org> (Alexey Gladkov's message of "Tue, 21 Dec 2021 14:09:37 +0100")

Alexey Gladkov <legion@kernel.org> writes:

> On Sun, Dec 19, 2021 at 11:58:41PM -0600, Eric W. Biederman wrote:
>> Qian Cai <quic_qiancai@quicinc.com> writes:
>> 
>> > On Wed, Nov 24, 2021 at 04:49:19PM -0500, Qian Cai wrote:
>> >> Hmm, I don't know if that or it is just this platfrom is lucky to trigger
>> >> the race condition quickly, but I can't reproduce it on x86 so far. I am
>> >> Cc'ing a few arm64 people to see if they have spot anything I might be
>> >> missing. The original bug report is here:
>> >> 
>> >> https://lore.kernel.org/lkml/YZV7Z+yXbsx9p3JN@fixkernel.com/
>> >
>> > Okay, I am finally able to reproduce this on x86_64 with the latest
>> > mainline as well by setting CONFIG_USER_NS and KASAN on the top of
>> > defconfig (I did not realize it did not select CONFIG_USER_NS in the first
>> > place). Anyway, it still took less than 5-minute by running:
>> >
>> > $ trinity -C 48
>> 
>> It took me a while to get to the point of reproducing this but I can
>> confirm I see this with 2 core VM, running 5.16.0-rc4.
>> 
>> Running trinity 2019.06 packaged in debian 11.
>
> I still can't reproduce :(
>
>> I didn't watch so I don't know if it was 5 minutes but I do know it took
>> less than an hour.
>
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -209,6 +209,7 @@ void put_ucounts(struct ucounts *ucounts)
>
>         if (atomic_dec_and_lock_irqsave(&ucounts->count, &ucounts_lock, flags)) {
>                 hlist_del_init(&ucounts->node);
> +               ucounts->ns = NULL;
>                 spin_unlock_irqrestore(&ucounts_lock, flags);
>                 kfree(ucounts);
>         }
>
> Does the previous hack increase the likelihood of an error being
> triggered?

It doesn't seem to make a difference.  That makes sense as the kernel
address sanitizer is part of the kernel configuration required to
reproduce the issue.

Eric

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